Photoacoustic imaging method and device

ABSTRACT

A photoacoustic imaging method is provided that can prevent the surface of the subject from appearing in an image when a part to be observed at a position deeper than the surface of the subject is imaged. The photoacoustic imaging device includes a unit for outputting pulsed light toward the subject, and generating photoacoustic data by detecting photoacoustic waves emitted from the subject exposed to the light. The photoacoustic imaging device also includes a region detection unit that detects a near-surface region of the subject based on the photoacoustic wave detection signals, and a correcting unit that attenuates (which encompasses removing) information of the near-surface region found by the region detection unit when the part to be observed is displayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending U.S. application Ser. No.14/457,921 filed Aug. 12, 2014, which is a Continuation of PCTInternational Application No. PCT/JP2013/000675 filed on Feb. 7, 2013,which claims priority under 35 U.S.C. § 119(a) to Japanese PatentApplication No. 2012-028572 filed on Feb. 13, 2012 and Japanese PatentApplication No. 2013-010856 filed on Jan. 24, 2013. Each of the aboveapplications is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a photoacoustic imaging method, namely,a method for imaging a subject, such as a living tissue, based onphotoacoustic waves emitted from the subject when it is exposed to lightoutputted toward the subject.

The present invention also relates to a device that carries out thephotoacoustic imaging method.

Background Art

As disclosed in Japanese Unexamined Patent Publication Nos. 2005-021380and 2011-217767 (hereinafter, Patent Documents 1 and 2, respectively)and X. Wang et al., “A High-Speed Photoacoustic Tomography System basedon a Commercial Ultrasound and a Custom Transducer Array”, Proc. ofSPIE, Vol. 7564, pp. 756424-1-756424-9, 2010 (hereinafter, Non-PatentDocument 1), for example, a photoacoustic imaging device for imaging theinterior of a living body using the photoacoustic effect isconventionally known. With the photoacoustic imaging device, pulsedlight, such as pulsed laser light, is outputted toward the living body.In the interior of the living body exposed to the pulsed light, a livingtissue absorbs the energy of the pulsed light and the volume of theliving tissue expands due to heat, and acoustic waves (photoacousticwaves) are emitted. By detecting the photoacoustic waves with adetection means, such as an ultrasound probe, the interior of the livingbody can be visualized based on the obtained electric signals(photoacoustic signals).

The photoacoustic imaging device constructs an image based only onphotoacoustic waves emitted from specific absorptive substances, and istherefore suitable for imaging a specific tissue, such as blood vessels,in a living body.

With the photoacoustic imaging device, a tissue at a deeper positionthan the surface of the subject, such as blood vessels of a living body,can be imaged, as described above. To this end, it is necessary to set arelatively high intensity of the pulsed light so that the pulsed lightcan reach deep positions. In this case, however, near-surface parts ofthe subject (such as the epidermis and the body hair) exposed to thepulsed light emit photoacoustic waves, and a photoacoustic image of thenear-surface parts may be generated and displayed. The thus displayednear-surface parts may hinder observation of an intended part to beobserved, such as blood vessels, or may even hide the part to beobserved.

Patent Document 1 also discloses a method for solving this problem. Thismethod involves applying a Fourier transform in the spatial direction tophotoacoustic wave detection signals obtained with a plurality ofdetection elements to cut off spatial low-frequency components, applyingan inverse Fourier transform to the signals, and generating anddisplaying a photoacoustic image using the thus converted image signals.

DISCLOSURE OF THE INVENTION

The method disclosed in Patent Document 1, however, has a problem that apart to be observed, such as blood vessels, extending in a directionparallel to a direction in which the detection elements are arranged isalso removed. This method also has a problem that, in a case where aprobe that holds the detection elements is not pressed against thesubject and the surface of the subject is not parallel to the directionin which the detection elements are arranged, that is, where thedetection elements are used in a state where they are not in directcontact with the subject in water or in a gel-like material, forexample, or there is a transparent substance on the surfaces of thedetection elements, the effect of removing the surface of the subject isnot sufficient.

In view of the above-described circumstances, the present invention isdirected to providing a photoacoustic imaging method that can reliablyprevent a near-surface part of the subject from being clearly imaged.

The present invention is also directed to providing a photoacousticimaging device that is capable of carrying out the photoacoustic imagingmethod.

A first aspect of the photoacoustic imaging method according to theinvention is a photoacoustic imaging method comprising: outputting lightfrom a light source toward a part to be observed in a subject through asurface of the subject; obtaining photoacoustic wave detection signalsby detecting photoacoustic waves emitted from the part to be observedexposed to the light; imaging the part to be observed based on thephotoacoustic wave detection signals and displaying the part to beobserved on an image display means; detecting a near-surface region ofthe subject based on the photoacoustic wave detection signals; andattenuating information of the near-surface region when the part to beobserved is displayed.

The description “attenuating information of the near-surface region when. . . is displayed” encompasses removing the information in a case wherethe maximum degree of attenuation is applied (the same applies to thefollowing description).

In the first aspect of the photoacoustic imaging method, morespecifically, the photoacoustic wave detection signals are detected withrespect to a region extending from the surface of the subject toward adepth direction of the subject, and a region extending between aposition where a first extremum of differential values of thephotoacoustic wave detection signals is found and a position where anext extremum of the differential values is found may be detected as thenear-surface region.

Alternatively, in the first aspect of the photoacoustic imaging method,more specifically, the photoacoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and a region extending betweena position where a first extremum of differential values of thephotoacoustic wave detection signals is found and a position apart fromthe position where the first extremum is found by a predetermined lengthin the depth direction of the subject may be detected as thenear-surface region.

Still alternatively, in the first aspect of the photoacoustic imagingmethod, more specifically, the photoacoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and a region extendingbetween a position where a differential value of the photoacoustic wavedetection signal that first exceeds a predetermined threshold value isfound and a position where, after a first extremum of the differentialvalues is found after the predetermined threshold value is exceeded, anext extremum of differential values of the photoacoustic wave detectionsignals is found may be detected as the near-surface region.

Yet alternatively, in the first aspect of the photoacoustic imagingmethod, more specifically, the photoacoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and a region extendingbetween a position where a differential value of the photoacoustic wavedetection signal that first exceeds a predetermined threshold value isfound and a position apart from the position where the differentialvalue that first exceeds the predetermined threshold value is found by apredetermined length in the depth direction of the subject may bedetected as the near-surface region.

Yet alternatively, in the first aspect of the photoacoustic imagingmethod, more specifically, the photoacoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and a region extendingbetween a position where a first extremum of differential values of thephotoacoustic wave detection signals is found after a differential valueof the photoacoustic wave detection signal that first exceeds apredetermined threshold value is found and a position where a nextextremum of the differential values is found may be detected as thenear-surface region.

Yet alternatively, in the first aspect of the photoacoustic imagingmethod, more specifically, the photoacoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and a region extendingbetween a position where a first extremum of differential values of thephotoacoustic wave detection signals is found after a differential valueof the photoacoustic wave detection signal that first exceeds apredetermined threshold value is found and a position apart from theposition where the first extremum is found by a predetermined length inthe depth direction of the subject may be detected as the near-surfaceregion.

In the first aspect of the photoacoustic imaging method, it is desirablethat smoothing processing be applied to the photoacoustic wave detectionsignals before the differential values of the photoacoustic wavedetection signals are calculated.

In the first aspect of the photoacoustic imaging method, it is desirablethat smoothing processing be applied to the differential values used inprocessing for detecting the near-surface region before the processingfor detecting the near-surface region is performed.

A second aspect of the photoacoustic imaging method according to theinvention is a photoacoustic imaging method comprising: outputting lightfrom a light source toward a part to be observed in a subject through asurface of the subject; obtaining photoacoustic wave detection signalsby detecting photoacoustic waves emitted from the part to be observedexposed to the light; imaging the part to be observed based on thephotoacoustic wave detection signals and displaying the part to beobserved on an image display means; outputting acoustic waves toward thepart to be observed through the surface of the subject, and obtainingreflected acoustic wave detection signals by detecting reflectedacoustic waves reflected from the subject; detecting a near-surfaceregion of the subject based on the reflected acoustic wave detectionsignals; and attenuating information of the near-surface region when thepart to be observed is displayed.

In the second aspect of the photoacoustic imaging method, morespecifically, the reflected acoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and a region extending betweena position where the reflected acoustic wave detection signal begins toincrease from a minimum value and a position where a first local maximumvalue of the reflected acoustic wave detection signals is found may bedetected as the near-surface region.

Alternatively, in the second aspect of the photoacoustic imaging method,more specifically, the reflected acoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and a region extendingbetween two positions that are apart from a position where an averagevalue between a minimum value of the reflected acoustic wave detectionsignals and a first detected local maximum value of the reflectedacoustic wave detection signals is found by a predetermined length inthe depth direction of the subject and a predetermined length in anopposite direction from the depth direction of the subject may bedetected as the near-surface region.

Further, it is more preferred that the photoacoustic imaging method ofthe invention be applied to a case where the light outputted toward thesubject has a wavelength in the range from 700 to 850 nm.

A first aspect of the photoacoustic imaging device according to theinvention is a photoacoustic imaging device comprising: a light sourcethat emits light toward a part to be observed in a subject through asurface of the subject; a photoacoustic wave detection means thatobtains photoacoustic wave detection signals by detecting photoacousticwaves emitted from the part to be observed exposed to the light; animage display means that images the part to be observed based on thephotoacoustic wave detection signals and displays the part to beobserved;

a means that detects a near-surface region of the subject based on thephotoacoustic wave detection signals; and a correcting means thatattenuates information of the near-surface region when the part to beobserved is displayed.

In the first aspect of the photoacoustic device according to theinvention, the photoacoustic wave detection signals are detected withrespect to a region extending from the surface of the subject toward adepth direction of the subject, and, as the means that detects anear-surface region, for example, one that detects, as the near-surfaceregion, a region extending between a position where a first extremum (alocal maximum value or a local minimum value) of differential values ofthe photoacoustic wave detection signals is found and a position where anext extremum of the differential values is found may be applied.

Alternatively, the photoacoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and, as the means that detectsa near-surface region, one that detects, as the near-surface region, aregion extending between a position where a first extremum ofdifferential values of the photoacoustic wave detection signals is foundand a position apart from the position where the first extremum is foundby a predetermined length in the depth direction of the subject may beapplied.

Still alternatively, the photoacoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and, as the means thatdetects a near-surface region, one that detects, as the near-surfaceregion, a region extending between a position where a differential valueof the photoacoustic wave detection signal that first exceeds apredetermined threshold value is found and a position where a firstextremum of differential values of the photoacoustic wave detectionsignals is found after the differential value that first exceeds thepredetermined threshold value is found may be applied.

Yet alternatively, the photoacoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and, as the means that detectsa near-surface region, one that detects, as the near-surface region, aregion extending between a position where a differential value of thephotoacoustic wave detection signal that first exceeds a predeterminedthreshold value is found and a position apart from the position wherethe differential value that first exceeds the predetermined thresholdvalue is found by a predetermined length in the depth direction of thesubject may be applied.

Yet alternatively, the photoacoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and, as the means that detectsa near-surface region, one that detects, as the near-surface region, aregion extending between a position where a first extremum ofdifferential values of the photoacoustic wave detection signals is foundafter a differential value of the photoacoustic wave detection signalthat first exceeds a predetermined threshold value is found and aposition where a next extremum is found may be applied.

Yet alternatively, the photoacoustic wave detection signals are detectedwith respect to a region extending from the surface of the subjecttoward a depth direction of the subject, and, as the means that detectsa near-surface region, one that detects, as the near-surface region, aregion extending between a position where a first extremum ofdifferential values of the photoacoustic wave detection signals is foundafter a differential value of the photoacoustic wave detection signalthat first exceeds a predetermined threshold value is found and aposition apart from the position where the first extremum is found by apredetermined length in the depth direction of the subject may beapplied.

It is desirable that the first aspect of the photoacoustic deviceaccording to the invention comprise a means that applies smoothingprocessing to the photoacoustic wave detection signals before thedifferential values of the photoacoustic wave detection signals arecalculated.

It is desirable that the first aspect of the photoacoustic deviceaccording to the invention comprise a means that applies smoothingprocessing to the differential values used in processing for detectingthe near-surface region before the processing for detecting thenear-surface region is performed.

A second aspect of the photoacoustic imaging device according to theinvention is a photoacoustic imaging device comprising: a light sourcethat emits light toward a part to be observed in a subject through asurface of the subject; a photoacoustic wave detection means thatobtains photoacoustic wave detection signals by detecting photoacousticwaves emitted from the part to be observed exposed to the light; animage display means that images the part to be observed based on thephotoacoustic wave detection signals and displays the part to beobserved; a means that outputs acoustic waves toward the part to beobserved through the surface of the subject; a reflected acoustic wavedetection means that obtains reflected acoustic wave detection signalsby detecting reflected acoustic waves reflected from the subject; ameans that detects a near-surface region of the subject based on thereflected acoustic wave detection signals; and a correcting means thatattenuates information of the near-surface region when the part to beobserved is displayed.

In the second aspect of the photoacoustic imaging device according tothe invention, the reflected acoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and, as the means thatdetects a near-surface region, one that detects, as the near-surfaceregion, a region extending between a position where the reflectedacoustic wave detection signal begins to increase from a minimum valueand a position where a first local maximum value of the reflectedacoustic wave detection signals is found may be applied.

Alternatively, the reflected acoustic wave detection signals aredetected with respect to a region extending from the surface of thesubject toward a depth direction of the subject, and, as the means thatdetects a near-surface region, one that detects, as the near-surfaceregion, a region extending between two positions that are apart from aposition where an average value between a minimum value of the reflectedacoustic wave detection signals and a first detected local maximum valueof the reflected acoustic wave detection signals is found by apredetermined length in a depth direction of the subject and apredetermined length in an opposite direction from the depth directionof the subject may be applied.

In the photoacoustic imaging device of the invention, it is morepreferred that, as the light source, one that emits light having awavelength in the range from 700 to 850 nm be used.

According to the first aspect of the photoacoustic imaging method of theinvention, the near-surface region of the subject is found based on thephotoacoustic wave detection signals, and information of thenear-surface region is attenuated when the part to be observed isdisplayed. This allows reliably preventing the near-surface part of thesubject from being clearly imaged. The present inventors have foundthrough a study that the phenomenon where the near-surface part of thesubject is imaged tends to occur when the light outputted toward thesubject has a wavelength in the range from about 700 to 850 nm.Therefore, it is particularly effective to apply the method of theinvention to the case where light having a wavelength in theabove-described range is used.

Further, according to the second aspect of the photoacoustic imagingmethod of the invention, acoustic waves are outputted toward the part tobe observed through the surface of the subject, reflected acoustic wavedetection signals are obtained by detecting reflected acoustic wavesreflected from the subject, the near-surface region of the subject isdetected based on the reflected acoustic wave detection signals, andinformation of the near-surface region is attenuated when the part to beobserved is displayed. This also allows reliably preventing thenear-surface part of the subject from being clearly imaged.

The first aspect of the photoacoustic device of the invention includesthe means that detects the near-surface region of the subject based onthe photoacoustic wave detection signals, and the correcting means thatattenuates information of the near-surface region when the part to beobserved is displayed. Therefore, the device can carry out theabove-described first aspect of the photoacoustic imaging methodaccording to the invention.

The second aspect of the photoacoustic imaging device according to theinvention includes the means that outputs acoustic waves toward the partto be observed through the surface of the subject, the reflectedacoustic wave detection means that obtains reflected acoustic wavedetection signals by detecting reflected acoustic waves reflected fromthe subject, the means that detects a near-surface region of the subjectbased on the reflected acoustic wave detection signals, and thecorrecting means that attenuates information of the near-surface regionwhen the part to be observed is displayed. Therefore, the device cancarry out the above-described second aspect of the photoacoustic imagingmethod according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the schematic configuration of aphotoacoustic imaging device according to one embodiment of theinvention,

FIG. 2 is a schematic diagram for explaining one method for detecting anear-surface region of a subject with the device shown in FIG. 1,

FIG. 3 is a schematic diagram for explaining another method fordetecting the near-surface region of the subject with the device shownin FIG. 1,

FIG. 4 is a block diagram illustrating the schematic configuration of aphotoacoustic imaging device according to another embodiment of theinvention,

FIG. 5 is a schematic diagram for explaining one method for detectingthe near-surface region of the subject with the device shown in FIG. 4,

FIG. 6 is a schematic diagram for explaining another method fordetecting the near-surface region of the subject with the device shownin FIG. 4,

FIG. 7 is a schematic diagram for explaining the near-surface regiondetected with the device shown in FIG. 1,

FIG. 8 is a schematic diagram for explaining the near-surface regiondetected with the device shown in FIG. 4,

FIG. 9 is a block diagram illustrating part of the configuration of aphotoacoustic imaging device according to still another embodiment ofthe invention,

FIG. 10 is a block diagram illustrating the schematic configuration of aphotoacoustic imaging device according to yet another embodiment of theinvention,

FIG. 11 is a flow chart illustrating the flow of part of processingperformed by the device shown in FIG. 10, and

FIG. 12 is a schematic diagram for explaining a method for detecting thenear-surface region of the subject with the device shown in FIG. 10.

DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. FIG. 1 is a block diagramillustrating the basic configuration of a photoacoustic imaging device10 according to one embodiment of the invention. As one example, thephotoacoustic imaging device 10 is capable of obtaining both aphotoacoustic image and an ultrasound image, and includes an ultrasoundprobe 11, an ultrasound unit 12, a laser light source unit 13, and animage display means 14.

The laser light source unit 13 emits laser light with a centerwavelength of 756 nm, for example. The laser light source unit 13outputs the laser light toward the subject. It is desirable that thelaser light be guided to the probe 11 with a light guide means, such asa plurality of optical fibers, and be outputted toward the subject froma part of the probe 11.

The probe 11 outputs (transmits) ultrasound toward the subject anddetects (receives) reflected ultrasound that is reflected from thesubject and returns to the probe 11. To this end, the probe 11 includesa plurality of ultrasound transducers, which are one-dimensionallyarranged, for example. Further, the probe 11 detects, with theultrasound transducers, ultrasound (photoacoustic waves) emitted from apart to be observed in the subject that has absorbed the laser lightfrom the laser light source unit 13. The probe 11 detects thephotoacoustic waves and outputs photoacoustic wave detection signals,and also detects the reflected ultrasound and outputs reflectedultrasound detection signals.

It should be noted that, in a case where the above-mentioned light guidemeans is coupled to the probe 11, end portions of the light guide means,such as tip portions of a plurality of optical fibers, are arrangedalong the direction in which the ultrasound transducers are arranged,and the laser light is outputted toward the subject from each endportion. In the following description, the case where the light guidemeans is coupled to the probe 11 as described above is described as anexample.

To obtain a photoacoustic image or an ultrasound image of the subject,the probe 11 is moved along a direction that is almost perpendicular tothe one-dimensional direction in which the ultrasound transducers arearranged, thereby achieving two-dimensional scanning of the subject withthe laser light and the ultrasound. This scanning may be achieved bymoving the probe 11 manually by the operator, or more precisetwo-dimensional scanning may be achieved using a scanning mechanism.

The ultrasound unit 12 includes a receiver circuit 21, an AD conversionmeans 22, a reception memory 23, a data separation means 24, an imagereconstruction means 25, a detection and logarithmic conversion means26, and an image construction means 27. Output from the imageconstruction means 27 is inputted to the image display means 14, whichis faulted by a CRT or a liquid crystal display device, for example. Theultrasound unit 12 also includes a transmission control circuit 30, anda control means 31 that controls operations of the components, etc., ofthe ultrasound unit 12.

The receiver circuit 21 receives the photoacoustic wave detectionsignals and the reflected ultrasound detection signals outputted fromthe probe 11. The AD conversion means 22 is a sampling means, whichsamples the photoacoustic wave detection signals and the reflectedultrasound detection signals received by the receiver circuit 21, andconverts the signals into photoacoustic data and reflected ultrasounddata, respectively, which are digital signals. This sampling isperformed at a predetermined sampling cycle synchronously with anexternally-inputted AD clock signal, for example.

The laser light source unit 13 includes a Q-switched pulsed laser 32,which is formed by a Ti:Sapphire laser, an OPO (optical parametricoscillation) laser formed by a YAG laser with second harmonic pumping,an alexandrite laser, or the like, and a flashlamp 33, which is anpumping light source. To the laser light source unit 13, a light triggersignal that instructs to output light is inputted from the control means31. In response to the light trigger signal, the laser light source unit13 turns on the flashlamp 33 to pump the Q-switched pulsed laser 32.When the Q-switched pulsed laser 32 is sufficiently pumped by theflashlamp 33, for example, the control means 31 outputs a Q-switchtrigger signal. In response to the Q-switch trigger signal, theQ-switched pulsed laser 32 turns on the Q-switch to output pulsed laserlight having a wavelength of 756 nm.

The time taken for the Q-switched pulsed laser 32 to be sufficientlypumped after the flashlamp 33 is turned on can be estimated fromcharacteristics, etc., of the Q-switched pulsed laser 32. It should benoted that, in place of controlling the Q-switch by the control means 31as described above, the Q-switch may be turned on by the laser lightsource unit 13 after the Q-switched pulsed laser 32 is sufficientlypumped. In this case, a signal indicating that the Q-switch has beenturned on may be sent to the ultrasound unit 12.

The control means 31 inputs to the transmission control circuit 30 anultrasound trigger signal that instructs to transmit ultrasound. Inresponse to the ultrasound trigger signal, the transmission controlcircuit 30 causes the probe 11 to transmit ultrasound. The control means31 first outputs the light trigger signal, and then outputs theultrasound trigger signal. When the light trigger signal is outputted,the laser light is outputted toward the subject and the photoacousticwaves are detected. Then, when the ultrasound trigger signal isoutputted, the ultrasound is transmitted to the subject and thereflected ultrasound is detected.

Further, the control means 31 outputs to the AD conversion means 22 asampling trigger signal that instructs to start sampling. The samplingtrigger signal is outputted after the light trigger signal is outputtedand before the ultrasound trigger signal is outputted, or morepreferably at timing when the laser light is actually outputted towardthe subject. To this end, the sampling trigger signal is outputtedsynchronously with the output of the Q-switch trigger signal by thecontrol means 31, for example. In response to the sampling triggersignal, the AD conversion means 22 starts sampling of the photoacousticwave detection signals outputted from the probe 11 and received by thereceiver circuit 21.

After outputting the light trigger signal, the control means 31 outputsthe ultrasound trigger signal at timing when the detection of thephotoacoustic waves ends. At this time, the AD conversion means 22 doesnot stop the sampling of the photoacoustic wave detection signals andcontinues the sampling. In other words, the control means 31 outputs theultrasound trigger signal while the AD conversion means 22 continues thesampling of the photoacoustic wave detection signals. As the probe 11transmits ultrasound in response to the ultrasound trigger signal, theobject to be detected by the probe 11 is changed from the photoacousticwaves to the reflected ultrasound. The AD conversion means 22 continuessampling to sample the detected ultrasound detection signals, therebycontinuously sampling the photoacoustic wave detection signals and thereflected ultrasound detection signals.

The AD conversion means 22 stores in the common reception memory 23 thephotoacoustic data and the reflected ultrasound data obtained by thesampling. The sampled data stored in the reception memory 23 is thephotoacoustic data until a certain point of time, and is the reflectedultrasound data after the certain point of time. The data separationmeans 24 separates the photoacoustic data and the reflected ultrasounddata stored in the reception memory 23 from each other.

Now, generation and display of a photoacoustic image are described. Tothe data separation means 24 shown in FIG. 1, the photoacoustic dataobtained by outputting the pulsed laser light having a wavelength of 756nm toward the subject and the reflected ultrasound data, which are readout from the reception memory 23, are inputted. When a photoacousticimage is generated, the data separation means 24 inputs only thephotoacoustic data to the image reconstruction means 25. The imagereconstruction means 25 reconstructs data representing a photoacousticimage based on the photoacoustic data.

The detection and logarithmic conversion means 26 generates an envelopeof the data representing a photoacoustic image, and then applieslogarithmic conversion to the envelope to increase the dynamic range.The detection and logarithmic conversion means 26 inputs the processeddata to the image construction means 27. Based on the inputted data, theimage construction means 27 constructs a photoacoustic image about across section scanned with the pulsed laser light, and inputs the datarepresenting the photoacoustic image to the image display means 14 viathe correcting means 51. As a result, the photoacoustic image about thecross section is displayed on the image display means 14.

When there is no particular necessity for correction, the correctingmeans 51 inputs the data representing the photoacoustic image to theimage display means 14 without processing the data. In a case where aninstruction is given when it is desired to avoid the near-surface partof the subject appearing on the image display means 14, the correctingmeans 51 performs correction processing. The correction processing willbe described in detail later.

It should be noted that, as described previously, the subject may betwo-dimensionally scanned with the laser light by moving the probe 11,and a photoacoustic image that three-dimensionally displays a desiredpart, such as blood vessels, of the subject may be generated anddisplayed based on image data about a plurality of cross sectionsobtained by the two-dimensional scanning.

Further, an ultrasound image of the subject may be generated anddisplayed based on the reflected ultrasound data separated by the dataseparation means 24. The generation and display of the ultrasound imagemay be performed according to a conventionally known method, which isnot directly related to the invention and therefore is not described indetail. The ultrasound image and the photoacoustic image may bedisplayed with being superimposed one on the other.

It should be noted that, although ultrasound is used as the acousticwave outputted toward the subject to obtain a reflected ultrasound imagein this embodiment, the acoustic wave is not limited to ultrasound. Theacoustic wave may be one having an audible frequency, as long as thefrequency is appropriately selected depending on the subject,measurement conditions, etc.

In a case where a photoacoustic image of blood vessels, or the like, ata position deeper than the surface of the subject is generated anddisplayed, it is necessary to set a high light intensity so that thelaser light outputted from the laser light source unit 13 reaches thedeeper position in the subject. In this case, however, near-surfaceparts of the subject (such as the epidermis and the body hair) exposedto the laser light emit photoacoustic waves, and a photoacoustic imageof the near-surface parts may often be generated and displayed. In acase where the deep part, which is the intended part to be observed, isdisplayed with increased luminance, part of the displayed image issaturated, and the displayed near-surface parts may hinder observationof the blood vessels, which are the intended part to be observed.Further, in a case where a projection image is displayed, the displayednear-surface parts may hide the part to be observed located behind thenear-surface parts. The photoacoustic imaging device 10 of thisembodiment allows solving this problem. Now, this point is described indetail.

As shown in FIG. 1, the data separation means 24 is connected to aregion detection means 50, and the photoacoustic data outputted from thedata separation means 24 is also inputted to the region detection means50. The region detection means 50 detects a near-surface region of thesubject based on the inputted photoacoustic data. It should be notedthat, although the near-surface region is detected based on thedigitized photoacoustic data in this embodiment, the near-surface regioncan be detected based on the photoacoustic wave detection signals beforedigitized. In the invention, “detecting a near-surface region based onthe photoacoustic wave detection signals” encompasses detecting thenear-surface region of the subject based on the photoacoustic dataresulting from digitizing the photoacoustic wave detection signals.

Now, a specific procedure for detecting the near-surface region isdescribed with reference to FIG. 2. First, the region detection means 50differentiates the photoacoustic data for each linear region thatextends from the surface of the subject toward the depth direction(which refers to a direction in which the depth increases) of thesubject. Values of this photoacoustic data correspond to values of thephotoacoustic wave detection signals, and indicates intensities of thephotoacoustic waves. FIG. 2 shows an example of distribution of thedifferential values, where “Z” denotes the depth direction. As shown,the distribution of the differential values along the depth direction Zhas clear extrema P1 and P2 at the front side and the back side,respectively, of the skin tissue (of the epidermis, or the epidermisplus the upper dermis, for example) of the subject. Therefore, theregion detection means 50 detects a position Z0 where the first extremumP1 of the differential values toward the depth direction is found as theboundary between the subject and the air, i.e., the position of thesurface of the subject, and detects a region R extending from theposition Z0 to a position where the next extremum P2 is found as thenear-surface region.

In a case where a photoacoustic image about a certain cross section ofthe subject is generated and displayed, the region detection isperformed at a plurality of positions along the cross section, and atwo-dimensional near-surface region is detected. An example of thetwo-dimensional near-surface region is schematically shown in FIG. 7. InFIG. 7, “PA” denotes a photoacoustic image, “T” denotes the surface ofthe subject, “W” denotes the part to be observed in the interior of thesubject, and “RE” denotes the two-dimensional near-surface region.

The region detection means 50 inputs information of the thus foundnear-surface region RE to the correcting means 51 shown in FIG. 1. Thecorrecting means 51 removes, from the image data representing thephotoacoustic image outputted from the image construction means 27,image information representing the near-surface region RE indicated bythe above information, and appropriately interpolates the removed part.Then, the correcting means 51 inputs the resulting image data to theimage display means 14. On the image display means 14, the near-surfacepart of the subject is not displayed, and a photoacoustic image thatbasically shows only the part to be observed W is displayed.

It should be noted that, in place of completely removing the imageinformation about the near-surface region RE, as described above,attenuation processing for reducing the display luminance of thenear-surface region RE may be performed. In this case, a thin image ofthe near-surface part of the subject is displayed in the display image.Even in this case, the troublesome situation where discrimination of thepart to be observed is difficult due to the displayed near-surface partcan be prevented, when compared to a case where the near-surface part isclearly displayed. In the invention, “attenuating information of thenear-surface region when . . . is displayed” encompasses displaying animage with completely removing the information of the near-surfaceregion, as described above. In the case where the attenuation processingas described above is performed, it is preferred to display an imagewith enclosing the region RE by a dashed line or a solid line, as shownin FIG. 7, to indicate the position of the processed near-surface regionRE.

Further, in a case where a photoacoustic image is generated anddisplayed as a so-called 3D image display that shows a pseudothree-dimensional presentation of the part to be observed of thesubject, a three-dimensional near-surface region, which is a set oftwo-dimensional near-surface regions, as described above, can bedetected. In such a 3D image, the part to be observed, such as bloodvessels, at a deep position may be covered by parts of skin in thenear-surface region depending on the angle of display. In this case,observation of the intended part to be observed cannot be achieved.Applying the invention to such a case allows obtaining a particularlyhigh level of effect of improving diagnosis performance of aphotoacoustic image.

Next, another method for detecting the near-surface region RE isdescribed with reference to FIG. 3. It should be noted that the curveshown in FIG. 3 is the same as that shown in FIG. 2, and other elementsdenoted by the same symbols as those in FIG. 2 are the same elements asthose shown in FIG. 2. This method may be performed by the regiondetection means 50 shown in FIG. 1. The position Z0 where the firstextremum P1 of the differential values toward the depth direction isfound is detected as the boundary between the subject and the air, i.e.,the position of the surface of the subject, and the region detectionmeans 50 detects a region R extending between the position Z0 and aposition apart from the position Z0 by a predetermined length in thedepth direction of the subject as the near-surface region.

The predetermined length is preferably defined by the number of pixels,such as 400 pixels. A preferred length for the predetermined length canbe found experimentally or empirically. It is desirable that thepredetermined length be changeable, as appropriate, according to aninstruction inputted by the user of the device. This allows the user ofthe device to set an optimal thickness of the near-surface region forattenuating the display information while observing the photoacousticimage displayed on the image display means 14.

Next, another embodiment of the invention is described with reference toFIG. 4. In FIG. 4, elements that are equivalent to the elements shown inFIG. 1 are denoted by the same reference numerals, and explanationsthereof are omitted unless otherwise necessary (the same applieshereinafter).

In the device shown in FIG. 4, a region detection means 150 that detectsthe near-surface region from the digital reflected ultrasound dataoutputted from the data separation means 24 is applied, in place of theregion detection means 50 shown in FIG. 1. The region detection means150 first calculates moving average values of values of the reflectedultrasound data along the depth direction from the surface of thesubject (which correspond to moving average values of signal intensitiesof the reflected ultrasound detection signals along the same direction).The values of the reflected ultrasound data have reduced influence ofspecifications that is unique to ultrasound, and indicates aninterfacial boundary based on intensities of the reflected ultrasoundreflected from the subject.

FIG. 5 shows an example of distribution of the average values, where “Z”denotes the depth direction. As shown, the distribution of the averagevalues along the depth direction Z has a minimum value S1 for a gel orwater area outside the surface of the subject, and the value begins toincrease from the minimum value S1 at a position Z1 of the surface ofthe subject. The value gradually increases for an area where the skintissue is present, and begins to decrease for an area where the skintissue is no longer present. That is, a local maximum value S2 of theaverage values is found at a position Z2 around which the skin tissue isno longer present. Therefore, the region detection means 150 detects aregion R extending between the position Z1 and the position Z2 as thenear-surface region. It should be noted that the local maximum value S2is distributed over a certain range of depth, and the position Z2 may beset at any position within the range of depth.

In a case where a photoacoustic image about a certain cross section ofthe subject is generated and displayed, the region detection isperformed at a plurality of positions along the cross section, and atwo-dimensional near-surface region is detected. An example of thetwo-dimensional near-surface region is schematically shown in FIG. 8. InFIG. 8, “US” denotes an ultrasound image, “T” denotes the surface of thesubject, and “RE” denotes the two-dimensional near-surface region.

The region detection means 150 inputs information of the thus foundnear-surface region RE to the correcting means 51 shown in FIG. 4. Thecorrecting means 51 removes, from the image data representing thephotoacoustic image outputted from the image construction means 27,image information representing the near-surface region RE indicated bythe above information, and appropriately interpolates the removed part.Then, the correcting means 51 inputs the resulting image data to theimage display means 14. On the image display means 14, the near-surfacepart of the subject is not displayed, and a photoacoustic image thatbasically shows only the part to be observed W is displayed.

Also in this case, in place of completely removing the image informationabout the near-surface region RE, as described above, attenuationprocessing for reducing the display luminance of the near-surface regionRE may be performed. In this case, a thin image of the near-surface partof the subject is displayed in the display image. Even in this case, thetroublesome situation where discrimination of the part to be observed isdifficult due to the displayed near-surface part can be prevented, whencompared to a case where the near-surface part is clearly displayed.

Next, another method for detecting the near-surface region RE from thereflected ultrasound data is described with reference to FIG. 6. Itshould be noted that the curve shown in FIG. 6 is the same as that shownin FIG. 5, and other elements denoted by the same symbols as those inFIG. 5 are the same elements as those shown in FIG. 5. This method maybe performed by the region detection means 150 shown in FIG. 4. Theregion detection means 150 calculates an average value between theminimum value S1 and the local maximum value S2 of the average values ofthe reflected ultrasound data, and finds a position Z2 apart from aposition Z0 where the calculated average value is found by apredetermined length R2 in the depth direction of the subject and aposition Z1 apart from the position Z0 by a predetermined length R1 inthe opposite direction from the depth direction of the subject. Then theregion detection means 150 detects a region extending between theposition Z2 and the position Z1 as the near-surface region R.

The predetermined lengths R2 and R1 are preferably defined by thenumbers of pixels, such that the former is 300 pixels and the latter is100 pixels. Preferred lengths for the predetermined lengths R2 and R1can be found experimentally or empirically. It is desirable that thepredetermined lengths be changeable, as appropriate, according to aninstruction inputted by the user of the device. This allows the user ofthe device to set an optimal thickness of the near-surface region forattenuating the display information while observing the photoacousticimage displayed on the image display means 14.

Next, yet another embodiment of the invention is described withreference to FIG. 10. In the device shown in FIG. 10, a region detectionmeans 250 is applied, in place of the region detection means 50 shown inFIG. 1. The region detection means 250 detects the near-surface regionfrom the digital photoacoustic data having been subjected to thelogarithmic conversion, which is outputted from the detection andlogarithmic conversion means 26. Now, region detection processing,correction processing, etc., performed by the region detection means 250and the following means are described with reference to FIG. 11, whichshows the flow of the processing.

As the processing is started, the region detection means 250 firstsubstitutes data values that are not greater than a predeterminedthreshold value of the photoacoustic data outputted from the detectionand logarithmic conversion means 26 with zero value, in step S1. Morespecifically, in a case where the photoacoustic data before thelogarithmic conversion is resulted from sampling at a sampling frequencyof 40 MHz and a quantization bit rate of 12 bits, and the digitalphotoacoustic data after the logarithmic conversion has values rangingfrom 1 to 11, for example, the threshold value is set to around “4”.This processing removes noise components in the low luminance range.

Then, in step S2, the region detection means 250 applies smoothingprocessing to the photoacoustic data. Specifically, the smoothingprocessing is achieved by calculating a moving average for each set of21 points of the photoacoustic data in the depth direction of thesubject. Namely, for photoacoustic data of a given pixel, an averagevalue of photoacoustic data of 21 pixels including continuous 10 pixelsfrom the given pixel toward the shallower side of the subject,continuous 10 pixels from the given pixel toward the deeper side of thesubject, and the given pixel is calculated, and the calculated averagevalue is used as the photoacoustic data of the given pixel, therebyachieving the smoothing processing.

Then, in step S3, the region detection means 250 applies differentialprocessing to the smoothed photoacoustic data. As described previously,the differential processing is performed for photoacoustic data of eachlinear region that extends from the surface of the subject toward thedepth direction Z. FIG. 12 shows an example of distribution ofdifferential values resulting from the differential processing, in thesame manner as in FIG. 2. As shown in FIG. 12, the distribution ofdifferential values may include noise components N at positions on thelight source side of the surface of the subject (in the oppositedirection from the depth direction Z). In the case where a positionwhere the first extremum P1 of the differential values toward the depthdirection Z is found is detected as the surface of the subject position,as in the method explained using FIG. 2, if there are the noisecomponents N, the noise components N may cause a situation where aposition where an extremum of the noise components N is found isincorrectly detected as the surface of the subject position.

In this embodiment, such an incorrect detection is prevented. Namely, instep S4, the region detection means 250 applies smoothing processing tothe differentiated photoacoustic data by calculating a moving averagefor each set of 11 points in the depth direction, for example, and thendetects the surface position of the subject based on a predeterminedthreshold value in step S5. This detection of the surface position isachieved by regarding a position Z0 where a differential value thatfirst exceeds a predetermined threshold value Th is found, which isdetected with respect to a line toward the depth direction Z, as theboundary between the subject and an ultrasound coupler or an ultrasoundgel, i.e., the surface position of the subject. This detection of thesurface position is performed for a plurality of lines (for example,every line) toward the depth direction Z.

Then, in step S6, the region detection means 250 detects a region Rextending between the position Z0 and a position apart from the positionZ0 by a predetermined length in the depth direction of the subject asthe near-surface region, and the correcting means 51 deletes image dataabout the near-surface region from the image data carrying thephotoacoustic image sent from the image construction means 27. Then, instep S7, the correcting means 51 adjusts luminance and contrast so thatdiagnosis performance is not impaired after the deletion of image data.Further, in a case where a composite image of the above-describedultrasound image and the photoacoustic image is displayed, thecorrecting means 51 adjusts a color map showing each image portion, andthe resulting photoacoustic image is displayed on the image displaymeans 14 in step S8.

Also in this embodiment, image data about the near-surface region of thesubject is deleted from the image data carrying a photoacoustic image,as described above. Therefore, a photoacoustic image of the near-surfacepart is not generated and displayed, thereby allowing displaying aphotoacoustic image where an intended part to be observed, such as bloodvessels, is more visible.

It should be noted that a preferable value for the threshold value Thcan be found experimentally or empirically. As one example, in the casewhere the photoacoustic data before the logarithmic conversion isresulted from sampling at a sampling frequency of 40 MHz and aquantization bit rate of 12 bits, and the digital photoacoustic dataafter the logarithmic conversion has values ranging from 1 to 11, asdescribed previously, the threshold value Th may be set to “0.02”, forexample. Also, a preferable value for the width of the region R fordeleting the image data can be found experimentally or empirically. Asone example, the width of the region R may be set to correspond to 130pieces of data.

Although the region R extending across a predetermined length in thedepth direction of the subject is detected as the near-surface region inthis embodiment, a position where the next extremum P2 toward the depthdirection is found after the first extremum P1 of the differentialvalues is found may be detected as the back side of a skin tissue, orthe like, of the subject, and a region extending between a positionwhere a differential value that exceeds the threshold value Th is foundand the position where the extremum P2 is found may be detected as thenear-surface region.

Further, in this embodiment, the smoothing processing applied to thephotoacoustic data in step S2 shown in FIG. 11 and the smoothingprocessing applied to the differential data in step S4 allow preventingincorrect detection of the near-surface region due to noise componentsin each data.

It should be noted that, in order to prevent incorrect detection of thesurface position of the subject due to the noise components N as shownin FIG. 12, it is also effective to detect, as the surface position ofthe subject, a position where the first extremum P1 toward the depthdirection is found after a differential value that exceeds thepredetermined threshold value Th is found. Also in this case, a region Rextending across a predetermined length in the depth direction of thesubject from the position where the extremum P1 is found may be detectedas the near-surface region, or a range extending between the positionwhere the extremum P1 is found and the position where the next extremumP2 in the depth direction is found may be detected as the near-surfaceregion.

It should be noted that the photoacoustic imaging device and method ofthe invention are not limited to the above-described embodiments, andvarious modifications and changes made to the above-describedembodiments are also within the scope of the invention.

For example, the invention is also applicable to a photoacoustic imagingdevice and a photoacoustic imaging method where deconvolution processingis performed. FIG. 9 is a block diagram illustrating part of aphotoacoustic imaging device configured to perform deconvolutionprocessing. The configuration shown in FIG. 9 is inserted between theimage reconstruction means 25 and the detection and logarithmicconversion means 26 shown in FIG. 1, for example, and includes a lightdifferential waveform deconvolution means 40 and a correcting means 46connected downstream the light differential waveform deconvolution means40. The light differential waveform deconvolution means 40 includesFourier transform means 41 and 42, an inverse filter calculation means43, a filter application means 44, and an inverse Fourier transformmeans 45.

The light differential waveform deconvolution means 40 deconvolves, fromthe data representing a photoacoustic image outputted from the imagereconstruction means 25, a light pulse differential waveform obtained bydifferentiating a temporal waveform of light intensity of the pulsedlaser light outputted toward the subject. With this deconvolution,photoacoustic image data representing an absorption distribution isobtained.

Now, this deconvolution is described in detail. The Fourier transformmeans (first Fourier transform means) 41 of the light differentialwaveform deconvolution means 40 converts the reconstructed photoacousticimage data from the time domain signal into a frequency domain signal byapplying a discrete Fourier transform. The Fourier transform means(second Fourier transform means) 42 converts a signal obtained bysampling the light pulse differential waveform at a predeterminedsampling rate from the time domain signal into a frequency domain signalby applying a discrete Fourier transform. As an algorithm for theFourier transform, FFT can be used, for example.

In this embodiment, the sampling rate for sampling the light pulsedifferential waveform is the same as the sampling rate for sampling thephotoacoustic wave detection signals at the AD conversion means 22. Forexample, the photoacoustic wave detection signals are sampledsynchronously with a sampling clock of Fs=40 MHz, and the light pulsedifferential waveform is also sampled at a sampling rate of Fs_h=40 MHz.The Fourier transform means 41 applies a Fourier transform of 1024points, for example, to the photoacoustic image data obtained by thesampling at 40 MHz and outputted from the image reconstruction means 25.The Fourier transform means 42 applies a Fourier transform of 1024points to the light pulse differential waveform sampled at 40 MHz.

The inverse filter calculation means 43 calculates a reciprocal of theFourier transformed light pulse differential waveform as an inversefilter. For example, the inverse filter calculation means 43 calculates,as the inverse filter, conj(fft_h)/abs(fft_h)², where fft_h is a signalobtained by Fourier transforming a light pulse differential waveform h.The filter application means 44 applies the inverse filter calculated bythe inverse filter calculation means 43 to the photoacoustic image dataFourier transformed by the Fourier transform means 41. For example, thefilter application means 44 multiplies, for each element, the Fouriercoefficient of the photoacoustic image data by the Fourier coefficientof the inverse filter. By applying the inverse filter, the light pulsedifferential waveform is deconvolved from the frequency domain signal.The inverse Fourier transform means 45 converts the photoacoustic signalprocessed with the inverse filter from the frequency domain signal to atime domain signal using an inverse Fourier transform. By applying theinverse Fourier transform, a time domain absorption distribution signalis obtained.

By performing the above-described processing, a light differential termcan be removed from the photoacoustic wave detection signal in which thelight differential term is convolved, thereby allowing calculating anabsorption distribution from the photoacoustic wave detection signal. Byimaging such an absorption distribution, a photoacoustic imagerepresenting an absorption distribution image can be obtained.

The correcting means 46 corrects the data from which the light pulsedifferential waveform has been deconvolved to remove influence ofreception angle-dependent properties of the ultrasound transducers ofthe probe 11 from the data from which the light pulse differentialwaveform has been deconvolved. In addition to or in place of thereception angle-dependent properties, the correcting means 46 removesinfluence of an incoming light distribution on the subject from the datafrom which the light pulse differential waveform has been deconvolved.It should be noted that a photoacoustic image may be generated withoutperforming these corrections.

What is claimed is:
 1. A photoacoustic imaging device comprising: alight source that emits light toward a part to be observed in a subjectthrough a surface of the subject; a photoacoustic wave detection unitthat obtains photoacoustic wave detection signals by detectingphotoacoustic waves emitted from the part to be observed exposed to thelight; an image display that displays the part to be observed; and acontroller configured to: perform image processing on the part to beobserved based on the photoacoustic wave detection signals; detect anear-surface region of the subject based on a set of the photoacousticwave detection signals with respect to a depth direction of the subject;perform attenuation processing of information of the near-surface regionwhen the part to be observed is displayed; detect the photoacoustic wavedetection signals with respect to a region extending from the surface ofthe subject toward the depth direction of the subject; calculatedifferential values of the photoacoustic wave detection signals;determine a first extremum of the differential values; and determinewhere a differential value of the photoacoustic wave detection signalexceeds a first predetermined threshold value, wherein the detectednear-surface region is a region extending between a position where afirst extremum of differential values of the photoacoustic wavedetection signals is found after a differential value of thephotoacoustic wave detection signal that first exceeds the predeterminedthreshold value is found and a position where a second extremum of thedifferential values is found, wherein the position of the secondextremum is further apart from the surface of the subject than theposition of the first extremum, and wherein the first extremum is alocal maximal and the second extremum is a local minimum.
 2. Thephotoacoustic imaging device as claimed in claim 1, wherein thecontroller is further configured to perform smoothing processing to thephotoacoustic wave detection signals before the differential values ofthe photoacoustic wave detection signals are calculated.
 3. Thephotoacoustic imaging device as claimed in claim 1, the controller isfurther configured to perform smoothing processing to the differentialvalues used in processing for detecting the near-surface region beforethe processing for detecting the near-surface region is performed.
 4. Aphotoacoustic imaging device comprising: a light source that emits lighttoward a part to be observed in a subject through a surface of thesubject; a photoacoustic wave detection unit that obtains photoacousticwave detection signals by detecting photoacoustic waves emitted from thepart to be observed exposed to the light; an image display that displaysthe part to be observed; and a controller configured to: perform imageprocessing on the part to be observed based on the photoacoustic wavedetection signals; detect a near-surface region of the subject based ona set of the photoacoustic wave detection signals with respect to adepth direction of the subject; perform attenuation processing ofinformation of the near-surface region when the part to be observed isdisplayed; detect the photoacoustic wave detection signals with respecta region extending from the surface of the subject toward the depthdirection of the subject; calculate differential values of thephotoacoustic wave detection signals; determine a first extremum of thedifferential values; and determine where a differential value of thephotoacoustic wave detection signal exceeds a first predeterminedthreshold value, wherein the detected near-surface region is a regionextending between a position where the first extremum of differentialvalues of the photoacoustic wave detection signals is found after thedifferential value of the photoacoustic wave detection signal that firstexceeds the predetermined threshold value is found and a position of thesubject apart from the position where the first extremum is found by apredetermined length in the depth direction of the subject.
 5. Thephotoacoustic imaging device as claimed in claim 1, wherein thecontroller is further configured to perform smoothing processing to thephotoacoustic wave detection signals before the differential values ofthe photoacoustic wave detection signals are calculated.
 6. Thephotoacoustic imaging device as claimed in claim 4, wherein thecontroller is further configured to perform smoothing processing to thedifferential values used in processing for detecting the near-surfaceregion before the processing for detecting the near-surface region isperformed.